Translucent PCA Ceramic, Ceramic Discharge Vessel, and Method of Making

ABSTRACT

A translucent polycrystalline material suitable for use in ceramic discharge vessels for metal halide lamps is produced by sintering an alumina powder doped with a MgO sintering aid in a nitrogen atmosphere containing a partial pressure of a vapor phase carbon-containing species. The sintered polycrystalline alumina has a grain boundary phase containing aluminum, oxygen and nitrogen. The formation of the AL—O—N grain boundary phase is believed to facilitate the transport of nitrogen from entrapped pores during sintering. Preferably, the PCA is sintered in a carbon-element furnace under flowing ultra-high-purity nitrogen.

BACKGROUND OF THE INVENTION

Translucent polycrystalline alumina (PCA) ceramic has made possiblepresent-day high-pressure sodium (HPS) and ceramic metal halide lamps.The arc discharge vessels in these applications must be capable ofwithstanding the high temperatures and pressures generated in anoperating lamp as well be resistant to chemical attack by the fillmaterials.

In HPS lamps, the discharge vessels are tubular as shown in FIG. 2,whereas for ceramic metal halide lamps discharge vessels can range froma cylindrical shape to an approximately spherical shape (bulgy).Examples of these types of arc discharge vessels are given in EuropeanPatent Application No. 0 587 238 A1 and U.S. Pat. No. 5,936,351,respectively. A bulgy-shaped discharge vessel is shown in FIG. 1. Thebulgy shape with its hemispherical ends yields a more uniformtemperature distribution, resulting in reduced corrosion of the PCA bythe lamp fills.

In the past, some of the key elements in sintering polycrystallinealumina (PCA) to translucency involved the use of (1) a high-puritypowder, (2) a small concentration of a MgO sintering aid, and (3)sintering in an H₂-containing atmosphere. It has been reported in theliterature that air, N₂, He, and Ar atmospheres may not be used, but H₂,O₂, or vacuum did permit the attainment of translucency. This was due tothe solubility of the gases in the lattice and grain boundaries allowingentrapped gaseous species to diffuse to the surface. In a vacuumenvironment, or in a gaseous atmosphere that is soluble and diffusedrapidly in PCA, the sintering process is not kinetically limited, andpore-free microstructures are achieved. Later work has indicated thattranslucent alumina may be sintered in dissociated ammonia (25% N₂ -75%H₂) and even in a CO atmosphere. The sintering of alumina has beenreported in N₂—H₂ atmospheres containing as low as 2% hydrogen. Becauseof cost and safety issues, it would be desirable to eliminate the needto add hydrogen gas and use nitrogen gas only. However, a N₂ atmospherealone has not been able to produce translucent PCA

SUMMARY OF THE INVENTION

It has been discovered that polycrystalline alumina can be sintered totranslucency in a nitrogen gas atmosphere in a carbon-element furnace.As used herein, translucency means a total transmittance of at least 92%in the visible wavelength region from about 400 nm to about 700 nm.Preferably, the total transmittance of the discharge vessel according tothis invention is at least 95%.

The sintering results in the carbon-element furnace are drasticallydifferent from those using the W-element, Mo-shield or alumina tubemuffle furnace. Microstructural analysis of the PCA sintered in N₂ in acarbon-element furnace indicates the formation of a grain-boundaryAL—O—N phase which is believed to facilitate the transport of nitrogenfrom the entrapped pores. The formation of the grain-boundary AL—O—Nphase is believed to result from the combination of the nitrogenatmosphere and vapor phase carbon-containing species, in particular, C,CO and CO₂, emanating from the carbon furnace components. For example,the formation of aluminum oxynitride (either Al₇O₉N or Al₂₃O₂₇N₅) mayproceed by the following reaction:7/2 Al₂O₃+3/2 C+3/2 N₂→Al₇O₉N+3/2 CO

The partial pressure of carbon at aluminum oxide sintering temperaturesis estimated to be about 10⁹ atm. Preferably, the furnace atmosphereshould contain from about 1×10⁻¹² atm to about 1×10⁻⁷ atm of carbon. Thefurnace atmosphere may also contain partial pressures of CO, CO₂, H₂,CH₄ and C₂H₂. In particular, the partial pressures of these gases duringsintering is estimated as: 10⁻³ to 10⁻⁴ P_(CO); 10⁻⁶ to 10⁻⁷ atmP_(CO2); 10⁻³ atm P_(H2), 10⁻¹⁰ atm P_(CH4) and 10⁻⁷ atm P_(C2H2).Although it is preferred to sinter the PCA in a carbon-element furnace,a similar sintering atmosphere may be generated by other means, e.g.,using graphite trays or placing other sources of carbon in other typesof furnaces. However, the atmosphere may be more difficult to control infurnaces that contain W and Mo components because these metals readilygetter carbon.

It should be noted that the partial pressure of hydrogen expected fromoutgassing of the carbon furnace components, 10⁻³ atm, is more than anorder of magnitude less than the partial pressure of hydrogen previouslyknown to be required for sintering aluminum oxide to translucency in anitrogen-hydrogen mixed gas atmosphere. Preferably, the PCA dischargevessel is sintered in a nitrogen atmosphere containing less than about0.2% H₂ by volume. More preferably, the furnace atmosphere containsabout 1×10⁻⁹ atm of carbon and about 1×10⁻³ to about 1×10⁻⁴ atm CO.

A relatively pure nitrogen gas source is used to create the furnaceatmosphere. Preferably, the nitrogen gas source should contain no morethan about 0.005% by volume total impurities. More preferably, thenitrogen gas is an ultra-high-purity grade that is 99.999% nitrogen byvolume. The sintering temperature may be in the range from about 1800°C. to about 2000° C. and sintering times may range from about 1 hour toabout 70 hours. More preferably, the discharge vessels are sintered atfrom at about 1850° C. to about 1950° C. for about 4 to about 50 hours,and most preferably, at about 1900° C. for about 10 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a prior art bulgy-shapeddischarge vessel.

FIG. 2 is a cross-sectional illustration of a prior art HPS dischargevessel.

FIG. 3 is a back-scattered electron image of a polished cross section ofa PCA tube sintered under N₂ in a carbon-element furnace.

FIG. 4 is a nitrogen map of the polished cross section of FIG. 3. Thedark regions indicate the presence of nitrogen.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

FIG. 1 is a cross-sectional illustration of a conventional bulgy-shapedarc discharge vessel. The arc discharge vessel 21 has a ceramic body 23which is comprised of polycrystalline alumina. The body 23 defines anarc discharge cavity 25 and has two capillaries 27 extending outwardlyin opposite directions from the discharge cavity 25. A typical thicknessof the discharge cavity wall is about 0.8 mm. The capillaries aresuitable for receiving, and sealing therein, electrode assemblies (notshown) which provide a conductive path for supplying electric power tothe discharge vessel in order to strike and sustain an arc within thedischarge cavity.

FIG. 2 is a cross-sectional illustration of a conventional dischargevessel for HPS lamps. The discharge vessel 50 has a tubular body 53comprised of PCA. Annular plugs 60 comprised of PCA are sealed in eachend of the tubular body 53 thereby defining discharge chamber 51. Theaperture in the annular plugs is for receiving an electrode assemblywhich typically consists of a niobium feedthrough to which a tungstenelectrode is attached. The niobium feedthrough is frit sealed in theaperture after a sodium/mercury amalgam and a buffer gas has been addedto discharge chamber 51.

Ceramic discharge vessels formed from a high-purity, finely dividedaluminum oxide (alumina) powder may be consolidated by isopressing,extrusion, slip casting, gel casting, or injection molding. The MgOdopant is generally added to the alumina powder prior to consolidation.The details of various methods of manufacturing green ceramic bodies fordischarge vessels are described in, for example, European Patent No. 0650 184 B1 (slip casting), U.S. Pat. No. 6,399,528 (gel casting),International Patent Application No. WO2004/007397 A1 (slip casting) andEuropean Patent Application No. EP 1 053 983 A2 (isopressing).

The sintering method of this invention produces a translucent PCAceramic that has a second nitrogen-containing phase at grain boundaries.The phase comprises aluminum, oxygen and nitrogen and is believed to bean aluminum oyxnitride. This second phase is believed to facilitate thediffusion of nitrogen out of the entrapped pores making it possible tosinter the PCA to translucency in a N₂ atmosphere without having to addat least 2% hydrogen. The sintering method and the resulting translucentPCA are described in more detail in the following examples. However, itshould be understood that the present invention is by no meansrestricted to such specific examples.

EXAMPLES

A high purity (99.97% pure) Al₂O₃ powder is preferably used as thestarting powder to form the polycrystalline alumina discharge vessel.Forming methods may include isopressing, extrusion, injection molding,gel casting and slip casting. For straight tubes, isopressing orextrusion is preferred. For more complex shapes, injection molding, gelcasting or slip casting may be used. Preferred alumina powders are CR30Fand CR6 manufactured by Baikowski. CR30F contains ˜80% alpha-Al₂O₃ and˜20% gamma-Al₂O₃, while CR6 is 100% alpha-Al₂O₃. The crystallite sizesare about 0.05 micrometers with a mean specific surface area of 30 m²/gfor CR30F and 6 m²/g for CR6. The reported average particle size isabout 0.5 micrometers for both types. Sintering aides such as MgO, Y₂O₃and ZrO₂ are preferred. MgO is required to sinter the PCA totranslucency. Preferably, the amount of MgO is from about 100 ppm toabout 1000 ppm. The alumina powders may be doped with the sintering aidsby mixing the alumina powder in aqueous solutions of the precursors ofthe sintering aids. In order to form the green shape, the powders arecombined with an appropriate binder material such as polyvinyl alcohol,polyethylene glycol, methylcellulose, or a wax. Prefiring of the shapesis conducted at 850-1350° C. in air for 1-4 h to remove the binders.Discharge vessels of varying sizes (wattages) and their capillaries weremade.

Sintering was accomplished in a carbon-element furnace (Centorr Company,Model M10) under one atmosphere of flowing ultra-high-purity (UHP) grade(99.999%) nitrogen gas. More preferably, the UHP-grade nitrogen gascontains <1ppm CO or CO₂, <2ppm O2, <3ppm H₂O and <0.5ppm totalhydrocarbons. The furnace was a horizontal furnace containing graphiteelements and carbon fiber insulation. Prefired PCA parts are placed inan alumina boat with or without setter powder. Two types of setterpowders are used: aluminum oxynitride and alumina. The gas flow rate inthe furnace corresponded to a linear gas speed of about 0.02 m/s. Thesintering temperatures (˜1800-1920° C.) are reached by heating at a rateof about 8-16° C./min. The hold time at the sintering temperature is4-40 h.

An aluminum oxynitride setter powder bed is preferred to create apartial pressure of aluminum oxynitride so that the grain-boundaryaluminum oxynitride phase is retained, which then facilitates diffusionof nitrogen trapped inside pores. PCA parts embedded in the powder bedsintered to significantly higher transmittance than those not buried inthe bed. The aluminum oxynitride powder bed may be accomplished by using(1) aluminum oxynitride powder, (2) alumina powder that would graduallyform an aluminum oxynitride phase in flowing nitrogen in a carbonfurnace, or (3) a mixture of aluminum nitride and alumina powder whichwould then react to form aluminum oxynitride in the carbon furnace.

The total transmittance of the sintered part involved placing aminiature incandescent lamp or a fiber-optical source inside thesintered part and measuring the total amount of diffuse lighttransmitted and integrated over a sphere. The wavelength range for themeasurement was from about 400 nm to about 700 nm. The following tableprovides the total transmittance of various PCA parts that were sinteredin flowing N₂ in a carbon-element, carbon-fiber insulation furnace.Total No. Sample Sintering Aid Sintering Cycle Transmittance 1 150 Wcapillary tube 200 ppm MgO 1900° C. - 4 hours 93.0% 400 ppm ZrO₂ 1890°C. - 4 hours  20 ppm Y₂O₃ 1900° C. - 6 hours 2 150 W capillary tube 200ppm MgO 1900° C. - 4 hours 94.5% 400 ppm ZrO₂ 1890° C. - 4 hours  20 ppmY₂O₃ 1900° C. - 18 hours 3 150 W bulgy 200 ppm MgO 1900° C. - 4 hours96.7% 400 ppm ZrO₂ 1900° C. - 48 hours  20 ppm Y₂O₃ 4 150 capillary tube200 ppm MgO 1900° C. - 4 hours 98.0% 400 ppm ZrO₂ 1890° C. - 40 hours 20 ppm Y₂O₃ 5 400 W capillary tube 500 ppm MgO 1910° C. - 40 hours93.0% 1920° C. - 10 hours 6 250 W HPS tube 200 ppm MgO 1910° C. - 40hours 93.0% 400 ppm ZrO₂ 1920° C. - 10 hours  20 ppm Y₂O₃ 7  35 W bulgy500 ppm MgO 1910° C. - 40 hours 92.0% 1920° C. - 30 hours 8  70 W HPStube 500 ppm MgO 1920° C. - 10 hours 94.0% 350 ppm Y₂O₃ (Al₇O₉N setterpowder) 9  70 W HPS tube 500 ppm MgO 1920° C. - 10 hours 92.0% 350 ppmY₂O₃ (Al₂O₃ setter powder)

The microstructures of the PCA sintered according to the method of thisinvention were examined by optical microscopy and scanning electronmicroscopy (SEM) with energy dispersive x-ray analysis (EDXA). Themorphology of the grains on the as-sintered surface is highly etchedwith cleavage steps, making it difficult to measure the grain size.SEM/EDXA of the surface showed the presence of nitrogen in PCA,indicating the formation of aluminum oxynitride on the surface.

FIG. 3 is a back-scattered electron image of a polished cross section ofa PCA tube sintered under N₂ in a carbon-element furnace. FIG. 4 is anitrogen map of the same polished cross section by electron microprobeanalysis. The dark regions indicate the presence of nitrogen and clearlyshow the presence of a thin layer of a nitrogen-containing phase at thealumina grain boundaries.

While there have been shown and described what are present considered tobe the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims.

1. A sintered translucent ceramic article comprised of polycrystallinealumina containing an amount of MgO and having a grain boundary phasecontaining aluminum, oxygen and nitrogen.
 2. The ceramic article ofclaim 1 wherein the grain boundary phase is aluminum oxynitride.
 3. Theceramic article of claim 3 wherein the amount of MgO in thepolycrystalline alumina is from about 100 ppm to about 1000 ppm.
 4. Aceramic discharge vessel comprising a ceramic body comprised oftranslucent polycrystalline alumina containing an amount of MgO andhaving a grain boundary phase containing aluminum, oxygen and nitrogen.5. The ceramic discharge vessel of claim 4 wherein the grain boundaryphase is aluminum oxynitride.
 6. The ceramic discharge vessel claim 4wherein the amount of MgO in the polycrystalline alumina is from about100 ppm to about 1000 ppm.
 7. The ceramic discharge vessel of claim 4wherein the discharge vessel has a total transmittance of at least 95%.8. A method of sintering ceramic body to translucency, the methodcomprising: (a) placing a ceramic body comprised of aluminum oxide dopedwith MgO in a furnace containing a carbon source and a nitrogenatmosphere formed by a nitrogen gas source having a total impurity levelof no more than about 0.005% by volume; (b) sintering the ceramic bodyat a temperature from about 1800° C. to about 2000° C. to form asintered translucent ceramic body.
 9. The method of claim 8 wherein thefurnace atmosphere during sintering contains from about 1×10⁻¹² atm toabout 1×10⁻⁷ atm of carbon.
 10. The method of claim 8 wherein thefurnace atmosphere during sintering contains less than about 0.2%hydrogen by volume.
 11. The method of claim 9 wherein the furnaceatmosphere during sintering contains from about 1×10⁻³ atm to about1×10⁻⁴ atm CO.
 12. The method of claim 9 wherein the furnace atmosphereduring sintering contains less than about 0.2% hydrogen by volume. 13.The method of claim 8 wherein the ceramic body is sintered for about 1hour to about 70 hours.
 14. The method of claim 8 wherein the ceramicbody is sintered at a temperature from about 1850° C. to about 1950° C.for about 4 hours to about 50 hours.
 15. The method of claim 8 whereinthe ceramic body is sintered at about 1900° C. for about 10 hours. 16.The method of claim 8 wherein the furnace is a carbon-element furnaceand the carbon source is one or more components of the furnace.
 17. Themethod of claim 16 wherein the furnace atmosphere during sinteringcontains from about 1×10 ⁻¹² atm to about 1×10⁻⁷ atm of carbon.
 18. Themethod of claim 17 wherein the furnace atmosphere during sinteringcontains less than about 0.2% hydrogen by volume.
 19. The method ofclaim 18 wherein the ceramic body is sintered at a temperature fromabout 1850° C. to about 1950° C. for about 4 hours to about 50 hours.20. A method of sintering a ceramic body to translucency, the methodcomprising: (a) placing a ceramic body comprised of aluminum oxide dopedwith about 100 ppm to about 1000 ppm MgO in a carbon-element furnacecontaining a nitrogen atmosphere formed by a nitrogen gas source havinga total impurity level of no more than about 0.005% by volume; (b)sintering the ceramic body at a temperature from about 1800° C. to about2000° C. for about 1 hour to about 70 hours to form a sinteredtranslucent ceramic body, the furnace atmosphere during sinteringcontaining from about 1×10⁻¹² atm to about 1×¹⁰⁻⁷ atm of carbon and lessthan about 0.2% hydrogen by volume.